drm/atmel-hlcdc: set layer REP bit to enable replication logic

This bit enables replication logic to expand an RGB color less than 24
bits, to 24 bits, which is used internally for all formats.  Otherwise,
the least significant bits are always set to zero and the color may not
be what is expected.

Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1562686509-8747-1-git-send-email-joshua.henderson@microchip.com